This invention relates to the field of cooking ranges which include an oven cavity configured to carry out both conventional thermal cooking and microwave cooking, or both methods of cooking simultaneously. Such ranges and ovens present a variety of design and construction problems brought about by areas of incompatability between thermal cooking devices and microwave cooking devices.
Thermal cooking is traditionally controlled by controlling the variables of temperature and time. Microwave cooking, until recently, was controlled through the variable of time only. More recent microwave ovens permit the microwave power level to be varied as well. The control of oven temperature remains a critical variable in both thermal cooking and in combination cooking. The term "combination cooking" is intended to mean the cooking of food through the simultaneous application of both thermal and microwave energy.
In thermal ovens and ranges temperature is traditionally controlled through a thermostat which includes a bulb portion located in the oven cavity for sensing the temperature in the cavity and providing a control signal in response to the sensed temperature. The customary practice in the industry has been to manufacture the thermostat bulb outer casing from steel, which has the advantages of high melting point, workability, and durability.
When a steel thermostat bulb is subjected to the hot, moist atmosphere in an oven a coating of iron oxides form on the steel. This oxidation process has not been of particular significance in thermal ranges heretofore, but causes highly undesirable effects in combination cooking ranges.
The iron oxides formed on the conventional steel bulb have a relatively high loss tangent at microwave frequencies, causing the bulb to be heated to a significant degree when exposed to microwave energy. This effect in combination cooking causes the thermostat to be highly inaccurate or inoperable as the presence of microwave energy in the cavity causes a distorted thermostatic signal because of the induced heating of the thermostat bulb.
Prior efforts to solve the problem have been primarily directed to isolating the thermostat bulb from the microwave energy through the use of RF screens of various types, which techniques involve additional manufacturing steps and materials and may interfere with the thermal accuracy of the bulb.